The present third note describes diagnostic procedures, prognosis, prophylaxis, therapy, and forensic medical problems connected with HIV infection and acquired immunodeficiency syndrome (AIDS). All these are topics of the utmost public health importance which have elicited much interest both among scientists and the general public also during the recent international conference held in Florence last June. HIV infection and the associated syndromes are extremely worrying in view of their increasing diffusion and extreme severity. In spite of the wealth of data already available, as yet we know little about this infection. Besides, all attempts at treatment and organization of prevention have proved ineffective, the latter being rendered vain by the extreme geographic spread of the disease, especially in underdeveloped countries where economic resources and individual and collective public health education are lacking. The fact of having successfully informed the medical profession about these complex problems, even though within the limits of the readers of this journal, is a source of satisfaction to us.